The present invention relates generally to MR imaging and, more particularly, to parallel excitation by a transmit coil array to realize a desired excitation profile. The present invention further relates to a parallel excitation pulse design that reduces RF power deposition on a subject.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Spatially selective excitation is widely used in MR imaging to induce transverse magnetization while limiting the size of the signal-contributing volume. Slice-selective excitation, the most commonly used, confines the signal-contributing volume to a fixed slice and thereby simplifies spatial encoding during signal acquisition and reduces scan time. Multi-dimensional excitation that produces localization along more than one dimension has been used to further this reduction in scan time. Its applications include, for example, localized spectroscopy, reduced-FOV scan of a region of interest, imaging of a target anatomy of unique shape, and echo planar imaging (EPI) with a shortened echo train length. In addition, profile (flip, phase and frequency) control across a sizeable volume with selective excitation has been exploited to improve excitation profile fidelity in the presence of B0 inhomogeneity or gradient non-linearity, and to reduce susceptibility artifacts.
Selective excitation is commonly implemented with a single transmit coil that transmits across an entire volume and produces a relatively uniform B1 field, e.g., a birdcage coil. Highly efficient pulse algorithms have been developed for designing excitation pulses that suit such a configuration. Notwithstanding the advantages achieved by these pulse design tools, technical difficulties remain. Issues with excitation pulse duration, excitation profile accuracy, and RF power absorption (SAR: specific absorption rate; measure of the rate of absorption of RF energy in the body) represent some of the outstanding challenges in a variety of applications. Compared to 1D excitation, flexible profile control along multiple dimensions with 2D or 3D excitation entails intensified pulsing activity and often requires powerful gradients to keep pulse duration in check. This limitation hinders applications of multi-dimensional excitation on scanners with general-purpose gradients. Substantial subject-dependency of B1 field, resulting from increased wave behavior and source-subject interaction at high frequencies, may also contribute to the difficulty of excitation profile control. An elevated rate of RF power deposition at high frequencies represents yet another factor that has a significant impact on the design and application of RF transmit modules and/or excitation pulses.